Intravital Imaging Study on Photodamage Produced by Femtosecond Near-infrared Laser Pulses In Vivo

Challenges in "probing spectroscopic probes" for noninvasive simultaneous disease diagnosis

Noninvasive diagnosis of human diseases relies on the detection of molecular markers (probes) in a painless manner. Although extrinsic and intrinsic molecular markers are often used, intrinsic disease probes (molecular markers) are preferable because they are naturally present in our body, and deviation in their concentration from normal levels clearly indicates anomalies in human bodies, that is, diseases. In this study, we report noninvasive spectroscopic measurements of total haemoglobin (Hb), bilirubin, and the ratio of oxy- and deoxyhaemoglobin as disease markers for anaemia, jaundice, and oxygen deficiency, respectively, using a meticulously designed optical fibre probe. The challenges in designing the fibre probe for simultaneous noninvasive detection, including optical power, spectral density of the probing light, and resolution of the spectrometer, were found to be critical to accurate measurements. Finally, a fibre-less, highly portable, and low-cost prototype was developed and tested in human clinical trials for the diagnosis of diseases, and these results were compared with conventional techniques (blood tests).

Phototoxicity induced in living HeLa cells by focused femtosecond laser pulses: a data-driven approach

Nonlinear optical microscopy is a powerful label-free imaging technology, providing biochemical and structural information in living cells and tissues. A possible drawback is photodamage induced by high-power ultrashort laser pulses. Here we present an experimental study on thousands of HeLa cells, to characterize the damage induced by focused femtosecond near-infrared laser pulses as a function of laser power, scanning speed and exposure time, in both wide-field and point-scanning illumination configurations. Our data-driven approach offers an interpretation of the underlying damage mechanisms and provides a predictive model that estimates its probability and extension and a safety limit for the working conditions in nonlinear optical microscopy. In particular, we demonstrate that cells can withstand high temperatures for a short amount of time, while they die if exposed for longer times to mild temperatures. It is thus better to illuminate the samples with high irradiances: thanks to the nonlinear imaging mechanism, much stronger signals will be generated, enabling fast imaging and thus avoiding sample photodamage.

Visualising UV-A light-induced damage to plasma membranes of eye lens

An eye lens is constantly exposed to the solar UV radiation, which is considered the most important external source of age-related changes to eye lens constituents. The accumulation of modifications of proteins and lipids with age can eventually lead to the development of progressive lens opacifications, such as cataracts. Though the impact of solar UV radiation on the structure and function of proteins is actively studied, little is known about the effect of photodamage on plasma membranes of lens cells. In this work we exploit Fluorescence Lifetime Imaging Microscopy (FLIM), together with viscosity-sensitive fluorophores termed molecular rotors, to study the changes in viscosity of plasma membranes of porcine eye lens resulting from two different types of photodamage: Type I (electron transfer) and Type II (singlet oxygen) reactions. We demonstrate that these two types of photodamage result in clearly distinct changes in viscosity - a decrease in the case of Type I damage and an increase in the case of Type II processes. Finally, to simulate age-related changes that occur in vivo, we expose an intact eye lens to UV-A light under anaerobic conditions. The observed decrease in viscosity within plasma membranes is consistent with the ability of eye lens constituents to sensitize Type I photodamage under natural irradiation conditions. These changes are likely to alter the transport of metabolites and predispose the whole tissue to the development of pathological processes such as cataracts.

SyncRGB-FLIM: synchronous fluorescence imaging of red, green and blue dyes enabled by ultra-broadband few-cycle laser excitation and fluorescence lifetime detection

We demonstrate for the first time that an ultra-broadband 7 femtosecond (fs) few-cycle laser can be used for multicolor nonlinear imaging in a single channel detection geometry, when employing a time-resolved fluorescence detection scheme. On a multi-chromophore-labelled cell sample we show that the few-cycle laser can efficiently excite the multiple chromophores over a >400 nm two-photon absorption range. By combining the few-cycle laser excitation with time-correlated single-photon counting (TCSPC) detection to record two-photon fluorescence lifetime imaging microscopy (FLIM) images, the localization of different chromophores in the cell can be identified based on their fluorescence decay properties. The novel SyncRGB-FLIM multi-color bioimaging technique opens the possibility of real-time protein-protein interaction studies, where its single-scan operation translates into reduced laser exposure of the sample, resulting in more photoprotective conditions for biological specimens.

The Impact of Compressed Femtosecond Laser Pulse Durations on Neuronal Tissue Used for Two-Photon Excitation Through an Endoscope

Accurate intraoperative tumour margin assessment is a major challenge in neurooncology, where sparse tumours beyond the bulk tumour are left undetected under conventional resection. Non-linear optical imaging can diagnose tissue at the sub-micron level and provide functional label-free histopathology in vivo. For this reason, a non-linear endomicroscope is being developed to characterize brain tissue intraoperatively based on multiple endogenous optical contrasts such as spectrally- and temporally-resolved fluorescence. To produce highly sensitive optical signatures that are specific to a given tissue type, short femtosecond pulsed lasers are required for efficient two-photon excitation. Yet, the potential of causing bio-damage has not been studied on neuronal tissue. Therefore, as a prerequisite to clinically testing the non-linear endomicroscope in vivo, the effect of short laser pulse durations (40-340 fs) on ex vivo brain tissue was investigated by monitoring the intensity, the spectral, and the lifetime properties of endogenous fluorophores under 800 and 890 nm two-photon excitation using a bi-modal non-linear endoscope. These properties were also validated by imaging samples on a benchtop multiphoton microscope. Our results show that under a constant mean laser power, excitation pulses as short as 40 fs do not negatively alter the biochemical/ biophysical properties of tissue even for prolonged irradiation.

Generation of a triple-fluorescent mouse strain allows a dynamic and spatial visualization of different liver phagocytes in vivo

Resident and circulating immune cells have been extensively studied due to their almost ubiquitous role in cell biology. Despite their classification under the "immune cell department", it is becoming increasingly clear that these cells are involved in many different non-immune related phenomena, including fetus development, vascular formation, memory, social behavior and many other phenotypes. There is a huge potential in combining high-throughput assays - including flow cytometry and gene analysis - with in vivo imaging. This can improve our knowledge in both basic and clinical cell biology, and accessing the expression of markers that are relevant in the context of both homeostasis and disease conditions might be instrumental. Here we describe how we generated a novel mouse strain that spontaneously express three different fluorescence markers under control of well-studied receptors (CX3CR1, CCR2 and CD11c) that are involved in a plethora of stages of cell ontogenesis, maturation, migration and behavior. Also, we assess the percentage of the expression and co-expression of each marker under homeostasis conditions, and how these cells behave when a local inflammation is induced in the liver applying a cutting-edge technology to image cells by confocal intravital microscopy.